Method for the production of caprolactam

ABSTRACT

A process is provided for the preparation of caprolactam from a compound of formula (I):  
     NC—(CH 2 ) 5 —CO—R  (I) 
     in which R is a carboxamide, carboxylic acid or carboxylic acid ester group, wherein  
     a) a compound (I) or a mixture of such compounds, in the presence of ammonia and optionally a liquid diluent (VI), is hydrogenated with hydrogen in the presence of a catalyst (II) to give a mixture (III),  
     b) the hydrogen and the catalyst (II) are separated from the mixture (III) to give a mixture (IV), and  
     c) the mixture (IV), optionally in the presence of a liquid diluent (VII), is converted to caprolactam in the presence of a catalyst (V).

[0001] The present invention relates to a process for the preparation ofcaprolactam from a compound of formula (I):

NC—(CH₂)₄—CO—R  (I)

[0002] in which R is a carboxamide, carboxylic acid or carboxylic acidester group, wherein

[0003] a) a compound (I) or a mixture of such compounds, in the presenceof ammonia and optionally a liquid diluent (VI), is hydrogenated withhydrogen in the presence of a catalyst (II), wherein a metal selectedfrom the group comprising Fe, Ni, Co and Ru, or mixtures thereof, isused as the catalytically active component of the catalyst (II), to givea mixture (III),

[0004] b) the hydrogen and the catalyst (II) are separated from themixture (III) to give a mixture (IV), and

[0005] c) the mixture (IV), optionally in the presence of a liquiddiluent (VII), is converted to caprolactam in the presence of a catalyst(V).

[0006] Processes for the preparation of caprolactam from compounds offormula (I) are known per se.

[0007] Thus DP 915 568 describes the conversion of cyanovaleric acidesters on Raney cobalt at 120° C. and 200 bar to give caprolactam yieldsof 40-52%.

[0008] The conversion of cyanovaleric acid esters on Raney catalysts isalso described in J. Polym. Sci. Part B, Polymer Lett. 2 (1964) no. 5,pages 491-3. Caprolactam yields of 31-74% are obtained.

[0009] EP-A-23 751 describes the hydrogenation of cyanovaleric acidesters on Ru/Fe fixed bed catalysts to give caprolactam yields of 35%.

[0010] According to JP 305330, cyanovaleric acid esters are firsthydrogenated on Raney nickel or Raney cobalt and then thermolyzedwithout a catalyst to give caprolactam yields of 80-90%.

[0011] The disadvantage of these processes is that the yields orspace-time yields are too low to meet industrial demands.

[0012] It is an object of the present invention to provide a processwhich makes it possible to prepare caprolactam from compounds of formula(I) in a technically simple and economic manner, avoiding saiddisadvantages.

[0013] We have found that this object is achieved by the process definedat the outset.

[0014] According to the invention, the compound (I) used has the formula

NC—(CH₂)₄—CO—R  (I)

[0015] in which R is a carboxamide, carboxylic acid or carboxylic acidester group.

[0016] It is also possible to use mixtures of such compounds, these alsobeing referred to as the compound (I) in terms of the present invention.It is particularly preferred to use a single compound as the compound(I).

[0017] Suitable carboxamide groups are advantageously those of thegeneral formula —CO—NR¹R², in which R¹ and R² independently of oneanother are hydrogen or an organic radical, preferably C₁-C₄ alkyl. Theparticularly preferred carboxamide group is the group —CONH₂.

[0018] Suitable carboxylic acid ester groups are advantageously those ofthe general formula -CO-OR³, in which R³ is an organic radical,preferably C₁-C₄ alkyl. The particularly preferred carboxylic acid estergroup is the group —CO—OCH₃.

[0019] The preparation of compounds of formula (I) is generally known,for example from Reppe, Lieb. Ann. Chem. 596 (1955) 127, BE 850 113,EP-A-576976 or EP-A-1 107 947.

[0020] According to the invention, in step a), the compound (I), in thepresence of ammonia and optionally a liquid diluent (VI), ishydrogenated with hydrogen in the presence of a catalyst (II) to give amixture (III).

[0021] Advantageously, at least 0.1 kg, preferably 0.1 to 10 kg,particularly preferably 0.2 to 5 kg and very particularly preferably 0.5to 5 kg of ammonia should generally be used per kg of compound (I).

[0022] Step a) can advantageously be carried out in the presence of aliquid diluent (VI). Suitable diluents (VI) are preferably water ororganic diluents, for example C₄ to C₉ alkanols such as n-butanol,i-butanol or n-pentanol, ethers such as methyl tert-butyl ether ortetrahydrofuran, preferably aliphatic hydrocarbons such as n-hexane,cycloaliphatic hydrocarbons such as cyclopentane or cyclohexane, andespecially aromatic hydrocarbons such as benzene, toluene, o-xylene,m-xylene, p-xylene, ethylbenzene, i-propylbenzene or di-i-propylbenzene,as well as mixtures of such compounds, for example petroleum ethers. Thehydrocarbons can carry functional groups such as halogens, for examplechlorine, as in chlorobenzene.

[0023] A suitable catalyst (II) is advantageously a heterogeneouscatalyst, said catalyst (II) preferably being used as a fixed bedcatalyst. A metal selected from the group consisting of Fe, Ni, Co andRu, or mixtures thereof, is useful as the catalytically active componentof the catalyst (II).

[0024] Such components can be used as an unsupported catalyst, forexample as Raney nickel, Raney cobalt or iron obtained by reduction fromsynthetic or natural materials containing iron oxide, for examplemagnetite.

[0025] Such components can be used as a supported catalyst. Suitablesupported catalysts can advantageously be obtained by impregnating orspraying a support with a solution containing compounds of such metalsor mixtures thereof, or by spray drying a suspension containing asupport and compounds of such metals or mixtures thereof. Such supportedcatalysts can contain supports known per se, preferably aluminum oxide,silicon oxide, aluminosilicates, lanthanum oxide, titanium dioxide,zirconium dioxide, magnesium oxide, zinc oxide, zeolites or activatedcarbon, or mixtures thereof.

[0026] In step a), the proportion of compound (I), based on the sum ofthe starting components, namely compound (I), ammonia and diluent (VI),is advantageously 0.1 to 50% by weight, preferably 1 to 30% by weightand particularly preferably 2 to 20% by weight.

[0027] The reaction can advantageously be carried out in the liquidphase.

[0028] The reaction can advantageously be carried out at temperaturesgenerally of 40 to 250° C., preferably of 50 to 100° C. and particularlypreferably of 60 to 120° C. The pressure should generally range from 1to 350 bar, preferably from 25 to 250 bar.

[0029] The reaction of step a) yields a mixture (III).

[0030] In step b), the hydrogen and the catalyst (II) are separated fromthe mixture (III) to give a mixture (IV).

[0031] The catalyst (II) can be separated off in a manner known per se.

[0032] In the case of a suspension procedure, the catalyst (II) canadvantageously be separated off by filtration, especially through amembrane of appropriate pore size. In the case of a fixed bed procedure,the reaction mixture can advantageously be withdrawn from the reactor toleave the catalyst (II) in the reactor.

[0033] The hydrogen can be separated off in a manner known per se,preferably in a high pressure separator. The high pressure separator canadvantageously be followed by [text missing] for separating off anyremaining hydrogen.

[0034] All or part of the ammonia can advantageously be separated off instep b).

[0035] The separation can advantageously be carried out by distillation,especially at bottom temperatures of 60 to 220° C. and pressures of 1 to30 bar.

[0036] A mixture (IV) is obtained after step b).

[0037] According to the invention, in step c), a mixture (IV),optionally in the presence of a liquid diluent (VII), is converted tocaprolactam in the presence of a catalyst (V).

[0038] Step c) can advantageously be carried out in the presence of aliquid diluent (VII). Suitable diluents (VII) are preferably water ororganic diluents, for example C₁ to C₉ alkanols such as methanol,ethanol, n-propanol, isopropanol, n-butanol, i-butanol or n-pentanol,preferably aliphatic hydrocarbons such as n-hexane, cycloaliphatichydrocarbons such as cyclopentane or cyclohexane, and especiallyaromatic hydrocarbons such as benzene, toluene, o-xylene, m-xylene,p-xylene, ethylbenzene, i-propylbenzene or di-i-propylbenzene, as wellas mixtures of such compounds, for example petroleum ethers. Thehydrocarbons can carry functional groups such as halogens, for examplechlorine, as in chlorobenzene.

[0039] Advantageously, the diluents (VI) and the diluents (VII) can beidentical.

[0040] A suitable catalyst (V) is advantageously a heterogeneouscatalyst. The catalyst (V) can preferably be used as a fixed bedcatalyst.

[0041] Suitable catalysts (V) are acidic, basic or amphoteric oxides ofthe elements of main group II, III or IV of the periodic table, such ascalcium oxide, magnesium oxide, boron oxide, aluminum oxide, tin oxideor silicon dioxide in the form of pyrogenic silicon dioxide, silica gel,kieselguhr, quartz or mixtures thereof, and also oxides of metals ofsubgroups II to VI of the periodic table, such as amorphous titaniumdioxide in the form of anatase or rutile, zirconium dioxide,manganese-oxide or mixtures thereof. It is also possible to uselanthanide and actinide oxides such as cerium oxide, thorium oxide,praseodymium oxide, samarium oxide, a rare earth mixed oxide or mixturesthereof with the abovementioned oxides. Examples of other possiblecatalysts are:

[0042] vanadium oxide, barium oxide, zinc oxide, niobium oxide, ironoxide, chromium oxide, molybdenum oxide, tungsten oxide or mixturesthereof. Mixtures of said oxides with one another are also possible.Some sulfides, selenides and tellurides, such as zinc telluride, tinselenide, molybdenum sulfide, tungsten sulfide and the sulfides ofnickel, zinc and chromium, can also be used.

[0043] The abovementioned compounds can be doped with, or contain,compounds of main groups I and VII of the periodic table.

[0044] Other suitable catalysts which may be mentioned are zeolites,phosphates and heteropolyacids, as well as acidic and alkaline ionexchangers like Nafion.

[0045] Preferred catalysts are titanium oxide, aluminum oxide, ceriumoxide and zirconium dioxide, particularly preferred catalysts beingactivated aluminum oxides such as those disclosed e.g. by

[0046] In the reaction of step c), ammonia can also be present, but ispreferably absent.

[0047] In step c), the proportion of the compound formed from thecompound (I) in step a) (compound (VIII)), based on the sum of thiscompound (VIII) and the diluent (VII), is advantageously 0.1 to 50% byweight, preferably 1 to 30% by weight and particularly preferably 2 to20% by weight.

[0048] The reaction can advantageously be carried out in the liquidphase at temperatures generally of 140 to 320° C., preferably of 180 to300° C. and particularly preferably of 200 to 280° C. The pressureshould generally range from 1 to 250 bar, preferably from 5 to 150 bar.

[0049] The preferred pressure and temperature conditions here are thoseunder which the reaction mixture is in the form of a single homogeneousliquid phase.

[0050] The catalyst loadings generally range from 0.05 to 5, preferablyfrom 0.1 to 2 and particularly preferably from 0.2 to 1 kg of compound(VIII) per 1 catalyst volume per hour.

[0051] The reaction of step c) yields a mixture containing caprolactam.In addition to caprolactam, this mixture generally contains low-boilingcomponents, high-boiling components, compound (VIII), optionally ammoniaand diluent (VII).

[0052] In terms of the present invention, low-boiling components areunderstood as meaning compounds boiling below caprolactam andhigh-boiling components are understood as meaning compounds boilingabove caprolactam.

[0053] Caprolactam can be obtained from this mixture by methods knownper se, such as extraction or distillation. In the case of distillation,the work-up can advantageously be effected by fractional distillation inone or more, such as 2 or 3, distillation apparatuses.

[0054] Suitable apparatuses are those conventionally used fordistillation, for example those described in: Kirk-Othmer, Encyclopediaof Chemical Technology, 3rd ed., vol. 7, John Wiley & Sons, New York,1979, pages 870-881, such as sieve-plate columns, bubble-cap columns orpacked columns.

[0055] Preferably, any ammonia and diluent (VII) still present areappropriately separated from the mixture first. The high-boilingcomponents, the low-boiling components and any unreacted compound (VIII)can then be separated from the remaining mixture, individually ortogether, to give caprolactam.

[0056] Advantageously, all or part of the diluent (VII) obtained in thiswork-up can be recycled into step a) or step c).

[0057] Advantageously, all or part of any high-boiling and/orlow-boiling components obtained in this work-up can be recycled intostep c).

[0058] Advantageously, all or part of any unreacted compound (VIII)obtained in this work-up can be recycled into step c).

[0059] The caprolactam obtained in the process according to theinvention can be used in a manner known per se for the preparation ofindustrially important polymers such as polyamides.

EXAMPLES

[0060] a) Batch Hydrogenation of Methyl Cyanovalerate

[0061] Methyl cyanovalerate, solvent and ammonia as the liquid phasewere placed, as shown in table 1, in a 0.27 liter autoclave fitted witha wire basket insert containing 80 g of catalyst pellets, and with a gasinjection stirrer, and were reacted under the conditions given in saidtable. The appropriate reaction pressure was kept constant by thecontinuous metering of hydrogen.

[0062] The results are shown in table 1. TABLE 1 Reaction Educts [g]conditions Product yield SOLV T p t in [%] Catalyst MCV (79 g) NH₃ [°C.] [bar] [min] MAH AH CL Magnetite 8.8 MTBE 8.1 110 250 30 92.4 3.0 3.6Magnetite 17 toluene 8.1 110 250 35 96.7 1.8 1.6 Raney Ni 17 — 69 50 100220 91.0 5.9 0.3 Cobalt 17 — 69 90 250 30 93.8 3.7 0.9 Cobalt 17 — 69 50100 70 98.2 0.8 0.3 Cobalt 8.8 toluene 69 50 100 165 93.9 — 1.1 Cobalt8.8 toluene 8.1 50 100 160 94.0 0.6 1.2 Raney Co 17 — 69 50 100 45 91.71.1 1.1 Raney Co 8.8 toluene — 50 100 65 93.6 — 0.3 Hydro— 8.8 toluene8.1 50 100 22 90.5 3.9 3.4 talcite Raney Co 8.8 toluene 8.8 60 100 9098.1 1.0 0.6 Raney Co 8.8 toluene 19.6 60 100 90 98.8 0.8 0.3

[0063] b) Batch Cyclization of Methyl 6-Aminohexanoate to Caprolactam

[0064] 2 ml (1.6 g) of a 10% by weight solution of methyl6-aminohexanoate in toluene were converted, as shown in table 2, in a 5ml autoclave at 150° C., with or without 1 g of powdered catalyst.

[0065] The conversions of methyl 6-aminohexanoate to caprolactam in [%]are shown in table 2. TABLE 2 Catalyst prec. t [min] none SiO₂ Al₂O₃TiO₂ ZrO₂ MgO CaO 15 4.4 46.9 70.3 53.1 43.7 8.1 4.7 30 5.2 69.7 89.173.6 63.1 11.6 5.8 45 5.9 80.0 92.4 81.2 71.5 14.5 6.2 60 7.0 83.1 94.284.5 76.7 17.7 7.7 120 11.6 98.7 99.0 98.0 98.0 28.6 13.0

[0066] c) Continuous Hydrogenation of Methyl Cyanovalerate

[0067] 1. On a cobalt catalyst

[0068] In a 100 ml tubular reactor with a separate feed for eductsolution, ammonia and hydrogen, 100 ml of cobalt catalyst were activatedat 280° C. and the loading (in g MCV/ml catalyst/hour) and temperaturewere varied as shown in table 3 at a pressure of 200 bar and anammonia/MCV ratio of 1:1.

[0069] The results are shown in table 3. TABLE 3 MCV conversion MAHselectivity Loading T [° C.] [%] [%] 0.3 80 94.60 94.46 0.15 80 98.6498.63 0.2 80 95.31 95.31 0.2 90 98.46 98.08 0.2 100 99.36 99.04 0.2 12099.92 99.32

[0070] 2. On a Raney Cobalt Doped with Cr

[0071] In a 100 ml tubular reactor with a separate feed for eductsolution, ammonia and hydrogen, and containing 100 ml of Cr-doped RaneyCo, the loading (in g MCV/ml catalyst/hour) was varied as shown in table4 at a pressure of 80 bar, a temperature of 80° C. and an ammonia/MCVratio of 2:1.

[0072] The results are shown in table 4. TABLE 4 Loading MCV conversion[%] MAH selectivity [%] 0.5 64.80 64.77 0.4 80.30 80.30 0.3 89.50 89.500.25 98.50 98.61 0.2 99.60 99.56

[0073] d) Continuous Cyclization of Methyl 6-aminohexanoate toCaprolactam

[0074] 1. On a TiO₂ Catalyst

[0075] A 10% by weight solution of methyl 6-aminohexanoate in toluene,possibly containing water as shown in table 5, was converted on a TiO₂catalyst (1.5 mm pellets) in a 20 ml tubular reactor (length 90 cm,diameter 6 mm) at the reaction temperature and loading (in 5 kg/l/h)shown in table 5.

[0076] The results are shown in table 5. TABLE 5 T Water [% by MAHconversion CL selectivity Loading [° C.] weight] [%] [%] 0.5 200 0 95.4494.54 0.4 200 0 97.73 95.24 0.5 220 0 98.30 96.85 0.5 220 0 98.64 96.230.5 220 5 98.93 96.44 0.5 250 0 97.56 94.95 0.5 240 0 96.87 95.85 0.5230 0 97.42 97.31 0.5 220 0 97.92 96.58

[0077] 2. On a Gamma-Aluminum Oxide Catalyst

[0078] A 10% by weight solution of methyl 6-aminohexanoate in toluene,possibly containing water as shown in table 5, was converted on agamma-aluminum oxide catalyst (1-3 mm chips) in a 20 ml tubular reactor(length 90 cm, diameter 6 mm) at the reaction temperature and loading(in kg/l/h) shown in table 6.

[0079] The results are shown in table 6. TABLE 6 T Water [% by MAHconversion CL selectivity Loading [° C.] weight] [%] [%] 0.5 220 0 97.8696.36 0.55 220 0 97.87 95.84 0.6 220 0 98.04 96.49

We claim:
 1. A process for the preparation of caprolactam from acompound of formula (I): NC—(CH₂)₄—CO—R  (I) in which r is acarboxamide, carboxylic acid or carboxylic acid ester group, wherein a)a compound (I) or a mixture of such compounds, in the presence ofammonia and optionally a liquid diluent (VI), is hydrogenated withhydrogen in the presence of a catalyst (II), wherein a metal selectedfrom the group comprising fe, ni, co and ru, or mixtures thereof, isused as the catalytically active component of the catalyst (II), to givea mixture (III), b) the hydrogen and the catalyst (II) are separatedfrom the mixture (III) to give a mixture (IV), and c) the mixture (IV),optionally in the presence of a liquid diluent (VII), is converted tocaprolactam in the presence of a catalyst (V).
 2. A process as claimedin claim 1 wherein the catalyst (II) is a heterogeneous catalyst.
 3. Aprocess as claimed in claim 1 or 2 wherein the catalyst (II) is a fixedbed catalyst.
 4. A process as claimed in any of claims 1 to 3 whereinthe reaction mixture used in step a) is in liquid form during thehydrogenation.
 5. A process as claimed in any of claims 1 to 4 whereinall or part of the ammonia is additionally separated off in step b). 6.A process as claimed in any of claims 1 to 5 wherein the catalyst (V)contains a metal oxide as the catalytically active component.
 7. Aprocess as claimed in any of claims 1 to 6 wherein the catalyst (V) is afixed bed catalyst.
 8. A process as claimed in any of claims 1 to 7wherein a metal oxide selected from the group comprising titaniumdioxide, aluminum oxide, cerium oxide and zirconium oxide, or mixturesthereof, is used as the catalytically active component of the catalyst(V).